Weather

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fxus63 kdtx 070737 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
337 am EDT sun Sep 7 2008 


Short term...today 


Upper level pattern remains characterized by broad upper level 
troughing anchored across the central northern Continental U.S....with weaker 
shortwaves ejecting out of this mean trough. Water vapor loop 
showing a lead impulse now working into northern Illinois. Enhanced 
upward vertical motion supplied by this wave and within the left 
exit region of a 120kt upper jet core helping to focus an area of 
rain stretching from southwest lower Michigan back into portions of Iowa. 
Based on radar trends...expect the leading edge of this to begin 
working into southeast lower Michigan within the next several hours. 
Models still indicating a modest amount of middle level fgen support 
and 850-700mb moisture advection through the morning hours... 
especially across the southern half of the forecast area. 
Thus...likely probability of precipitation remain warranted...and based on trends will 
expand the higher probability of precipitation northward to include the I-69 corridor. 
Theta-E fields do suggest a narrow ribbon of elevated instability... 
while recent lightning data depicted a negative strike in a cell 
coming off Lake Michigan. Thus...will hang on to the mention of 
isolated thunder. 


Precipitation will quickly taper off by midday as subsidence 
develops in the wake of the shortwave...allowing for some clearing 
late in the day. However...good temperature recovery will be 
problematic today given the limited insolation early on. 12z NAM 
thermal forecasts may be too pessimistic with highs only in the 60s 
across the south...but will trend slightly cooler with readings 
struggling to reach much above the lower 70s. 


&& 


Long term... 


Deepening upper wave/shortwave trough prognosticated to come out of the 
northern rockies...tracking through the central Great Lakes on 
Tuesday. With a deeper and more amplified solution...the trend has 
been to slow things down...which could result in the daytime hours 
of Monday remaining dry as shortwave ridging will be in place 
through the day. Still...big increase in isentropic ascent late in 
the day supports low chance probability of precipitation before the lower atmosphere 
saturates enough to support heavier rain Monday night into Tuesday 
morning. Rapid cyclogenesis to take place within the tight 
baroclinic zone over the southern Great Lakes...with the actual 
surface low forecasted to track through Detroit or near the Ohio 
border...per 00z UKMET/NAM/Canadian global. The GFS looks to be 
overdeveloped and too far north with the surface low. In any 
event...the more active region looks to be associated with the 850 
mb fgen across the northern third of the County Warning Area...per 00z NAM. Have 
raised probability of precipitation to categorical across the north half of the County Warning Area...with 
continued mention of thunder as showalter index shown going 
negative. 


Tuesday is continuing to shape up as a very cool day...a first taste 
of fall...as low level cold advection occurs through the 
day...offsetting any possible afternoon insolation/clearing...with 
maxes holding in the 60s based on 850 mb temperatures hovering near 4 c. 
With clear/mostly clear skies Tuesday night and large surface high 
pressure building into Southeast Michigan...looking for mins to fall 
into the 40 to 45 degree range across most of the County Warning Area. 


Will need nearly full sunshine to get back into the lower 70s on 
Wednesday as the high/ridging will be slow to exit...limiting the 
depth of mixing and diurnal swing. 


Good return flow does look to kick in Wednesday night into 
Thursday...with the better defined warm front remaining west of the 
County Warning Area...and will thus continue with a dry forecast Wednesday night. 
Not real thrilled with the precipitation chances for Thursday...per 
00z GFS/Canadian global...but will maintain low chance probability of precipitation in case 
the frontal boundary/surface low is displaced farther southeast...as 
indicated by 12z European. 


&& 


Marine... 


Winds will remain predominately out of the southwest through early 
Monday...as high pressure holds south of the region. While this 
will generally promote favorable boating conditions across the 
marine areas...one troublesome spot may be across Saginaw Bay as a 
favorable wind direction moving across the longer fetch generates 
higher waves. Latest model guidance suggests waves will increase 
to around 3 feet under this flow...so will hold off on issuing any 
advisories. However...trends will be closely monitored for any 
indication of larger wave heights that would necessitate an 
advisory. 


Winds will strengthen Monday night and Tuesday in response to a low 
pressure system moving through the lakes region. The potential 
exists for winds to approach gale force during this time...prompting 
the issuance of a gale watch for most of the Lake Huron open waters 
as well as from outer Saginaw Bay through The Harbor beach nearshore 
area. Waves will remain high well into Tuesday night as colder air 
moves across the warmer lake waters. Winds and waves will subside 
quickly through late Wednesday as high pressure builds back into the 
region. 


&& 


Previous discussion...issued 116 am EDT sun Sep 7 2008 


Aviation... 


An area of rain currently moving across north central Illinois is 
associated with an elevated frontal boundary. This system will track 
across southern Michigan during most of the morning. Lingering low 
level dry air should initially inhibit much precipitation from reaching the 
surface. However...persistent large scale forcing should allow rain 
to develop at or shortly after 09z. Model soundings indicate that 
moistening of the low levels will result in MVFR ceilings and visibility in 
rain by 12z...persisting to 15z. Conditions will quickly improve 
after 16z as large scale subsidence builds across Southeast Michigan in the wake 
of this system. 


&& 


DTX watches/warnings/advisories... 
Michigan...none. 


Lake Huron... 
gale watch...lhz362-lhz363-lhz421-lhz441-lhz462-lhz463- 
lhz464...from 4 am Tuesday to 2 PM Tuesday. 


Lake St Clair...none. 
Michigan waters of Lake Erie...none. 


&& 


$$ 


Short term...mr 
long term....sf 
marine.......mr 
aviation.....SC 




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